1. Field of the Invention
The present invention relates to the field of thermal heads to be used in thermal printing, and in particular, to a heat accumulation compensation method and improvement of related apparatus wherein compensation for the heat accumulation is performed taking into account the effects of heat accumulation in adjacent heating elements on a heating element currently heating printing medium.
2. Description of the Prior Art
In a conventional thermal head to be used for the thermal printing, an array of a multiplicity of heating elements are normally arranged in the main scan direction of the thermal printing medium such as a thermal printing paper and an ink donor sheet so as to corresponds to the number of picture elements in one scan line, and colors are caused to develop in the thermal printing medium which is, in slidingly contact with the heating parts of the heating elements, causing relevant heating elements to heat the medium corresponding to the picture image information.
In printing with such thermal head, effects of heat accumulation on each heating element varies according to the manner in which the image information is applied. That is, for example, when a heating element has been heated continuously in previous lines, the printing of data in the next line starts while this particular heating element does not become cool completely. On the other hand, when a heating element has not been heated for a long time, the printing of data of the next line starts with the heating element being completely cool. As a result the print density (shade level) varies in the above two cases lowering the quality of the printed picture image. Such phenomenon is particularly remarkable when a high speed printing is performed in which the printing time is less than 10 msec per line.
In order to cope with such problem, the prior art controls the width of a pulse (hereinafter called heating pulse) or voltage to be applied to heating elements currently performing printing to energize these elements. For example, when a heating element has been energized in the previous line, the width of a heating pulse is shortened when printing the current line.
However, in such prior art heat accumulation compensation system, a heating element is subject to heat accumulation compensation independently from other heating elements and the effect of the heat accumulation for heating elements adjacent to the heating element are not taken into account, making the prior art heat accumulation compensation unsatisfactory. Particularly, in the thermal printing of the transferring type which uses ink donor sheets as a printing medium, effect from heat accumulation in the adjacent heating elements is increased due to thermal diffusion on the ink donor surface, and favorable printing could not be effected.